This paper uses Raymond Boudon's model of educational expansion to examine the relationship between education and social mobility, paying particular attention to post-compulsory education--an important site of social differentiation in England. The paper shows how Boudon focuses explicitly on the consequences of educational expansion, and argues that his work helps us understand why widening access to post-compulsory education does not necessarily lead to higher rates of social mobility. We investigate Boudon's key theoretical insights and assess the contemporary relevance of his model. The paper argues that the fundamental assumptions of Boudon's model not only remain valid, but have been intensified by systemic changes in English post-compulsory education, and its articulation with the labour market.